The present invention generally relates to a converter control device, and particularly to a converter control device including an arrangement in which a plurality of converters are connected together in parallel, the converters being connected between a first power source and a second power source, and each having a plurality of switching elements and reactors to perform a voltage conversion in both directions, the converter control device changing the number of converter phases to be driven according to converter passing power.
In power supply systems using fuel cells, a voltage converter raising or dropping the voltage of an output from a secondary battery is provided and connected to an output terminal of the fuel cells to supply power. The purpose of the provision of the voltage converter is to deal with a fluctuation in load exceeding the power generation capacity of the fuel cells, to improve system efficiency, and to recover regenerative power when a regenerative motor is utilized as a load. Here, the voltage converter has a DC voltage converting function and is sometimes referred to as a DC/DC converter. For example, the voltage converter is composed of switching elements and a reactor. For example, to reduce the rating capacity of the switching elements, a plurality of converters are connected together in parallel.
For instance, Japanese Patent Laid-Open Publication No. 2006-33934 discloses that in order to deal with a rapid change in load amount exceeding the charging capability of a fuel cells, a voltage converter operating in a plurality of phases is connected between the fuel cells and a battery so that the number of phases of the voltage converter and duty ratio, can be varied based on a predicted variation in load amount. In Japanese Patent Laid-Open Publication No. 2006-33934, it is described as follows. In the voltage converter including the plurality of phases, generally, loss power lost in the converter varies depending on the value of passing power corresponding to the quantity of I/O conversion energy or the quantity of operational work. With high passing power, the loss is lower for a three-phase operation than for the single-phase operation. With low passing power, the loss is lower for the single-phase operation than for the three-phase operation. The reason for the variation in loss power is described as follows in Japanese Patent Laid-Open Publication No. 2006-33934. Losses to the three-phase bridge converter include a reactor copper loss caused by coils of the reactors, a module loss caused by operation of the switching elements, and a reactor iron loss caused by a magnetic substance in the reactors. The reactor copper loss and the module loss increase consistently with increase of the passing power and are heavier for the single-phase operation than for the three-phase operation. The reactor iron loss is almost independent of the passing power and is heavier for the three-phase operation than for the single-phase operation. Japanese Patent Laid-Open Publication No. 2006-33934 further describes the following. The single-phase operation is performed in a region of low passing power, whereas the three-phase operation is performed in a region of high passing power. When the three-phase operation is changed to the single-phase operation, the effective value of alternating current fluctuates in connection with a voltage conversion. Thus, in PID control, the voltage, current, and power fluctuate temporarily. Accordingly, the shortage of the power is compensated for by increasing the duty ratio.
Furthermore, Japanese Patent Laid-Open Publication No. 2003-235252 discloses a method of maximizing a conversion efficiency when a plurality of DC/DC converters are provided between an inverter and a battery. According to Japanese Patent Laid-Open Publication No. 2003-235252, the method uses master-slave DC/DC converters including one master DC/DC converter. The number of DC/DC converters to be operated, including the master DC/DC converter, is set with input power to or output power from the master DC/DC converter defined as reference power. Then, the output voltage from the master-slave DC/DC converters is increased and reduced within not exceeding the maximum allowable charging voltage and current of the battery. Then, the conversion efficiency is calculated, and the output voltage is adjusted to a value almost corresponding to the maximum conversion efficiency. Japanese Patent Laid-Open Publication No. 2003-235252 also describes the following. The conversion efficiency of the DC/DC converters is related to a switching loss on the primary side and a loss on the secondary side which is caused by a forward voltage drop in a rectifying diode. With high input power, the primary-side loss increases. With low input power, the primary-side loss decreases, and the secondary-side loss becomes dominant.
Japanese Patent Laid-Open Publication No. 2003-111384 discloses a method of, when the voltage of power of a main power source is converted by a plurality of DC/DC converters connected in parallel and the converted voltage is supplied to an auxiliary battery, preventing a possible increase in the use frequency of a particular DC/DC converter. Japanese Patent Laid-Open Publication No. 2003-111384 describes the following. The order in which the plurality of DC/DC converters are started is varied according to a predetermined specified order. The predetermined specified order is set according to the contents of measured voltage-current characteristics of the DC/DC converters.
As described above, in the configuration using the plurality of converters connected in parallel, the number of converter phases to be driven is controllably changed depending on the passing power through the converters. Furthermore, switching duty ratio is controlled in order to perform a voltage conversion so as to achieve a desired voltage rise or drop. In this case, for example, feedback control is used which feeds back the measured value of duty ratio of an actual operation with respect to a instruction value of the duty ratio to inhibit a deviation between the measured value and the instruction value of the duty ratio. For instance, a PID (Proportional-Integral-Derivative) control system can be used for the feedback control of the duty ratio.
When the number of converter phases used for the voltage conversion is changed, the state of a relevant feedback loop may change. As a result, the feedback control is not always optimum unless some appropriate measures are taken. For example, in the case where the number of converter phases is changed when the converter passing power exhibits a certain value, a passing current per converter circuit changes, resulting in a corresponding change in a correction value of an integration term for PID control. Thus, since a change in the number of converter phases may change the state of the feedback loop, the feedback control is not always optimum unless some appropriate measures are taken.